This application claims priority to a U.S. National Stage Application of Patent Cooperation Treaty (PCT) of PCT/EP2014/001381, filed May 22, 2014, which claims priority to European Patent Application 13305674.7, filed May 24, 2013.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for treating subterranean formations, in particular, compositions and methods for cementing and completing wells in which hydrogen sulfide is present.
During the construction of subterranean wells, it is common, during and after drilling, to place a tubular body in the wellbore. The tubular body may comprise drillpipe, casing, liner, coiled tubing or combinations thereof. The purpose of the tubular body is to act as a conduit through which desirable fluids from the well may travel and be collected. The tubular body is normally secured in the well by a cement sheath. The cement sheath provides mechanical support and hydraulic isolation between the zones or layers that the well penetrates. The latter function is important because it prevents hydraulic communication between zones that may result in contamination. For example, the cement sheath blocks fluids from oil or gas zones from entering the water table and polluting drinking water. In addition, to optimize a well's production efficiency, it may be desirable to isolate, for example, a gas-producing zone from an oil-producing zone.
The cement sheath achieves hydraulic isolation because of its low permeability. In addition, intimate bonding between the cement sheath and both the tubular body and borehole is necessary to prevent leaks. However, over time the cement sheath can deteriorate and become permeable. Alternatively, the bonding between the cement sheath and the tubular body or borehole may become compromised. The principal causes of deterioration and debonding include physical stresses associated with tectonic movements, temperature changes, pressure changes inside the casing and chemical deterioration of the cement.
Some oil and gas fields have formations whose fluids contain acid gases such as carbon dioxide and hydrogen sulfide. Such wells may be challenging from a zonal isolation point of view.
Oil and gas that contains elevated amounts of hydrogen sulfide are called “sour.” It has been estimated that 15 to 25% of natural gas in the United States may contain hydrogen sulfide. Worldwide, the percentage could be as high as 30%. Hydrogen sulfide is a toxic substance; therefore, it is important to prevent it from escaping through the cement sheath into aquifers or to the surface. Furthermore, hydrogen sulfide is corrosive to steel, and maintaining a competent cement sheath is essential to prevent casing deterioration during the life of the well.